LK-99 Isn't a Superconductor - How Science Sleuths Solved the Mystery

Researchers seem to have solved the puzzle of LK-99. Scientific detective work has unearthed evidence that the material is not a superconductor, and clarified its actual properties. Nature: The conclusion dashes hopes that LK-99 -- a compound of copper, lead, phosphorus and oxygen -- marked the discovery of the first superconductor that works at room temperature and ambient pressure. Instead, studies have shown that impurities in the material -- in particular, copper sulfide -- were responsible for the sharp drops in electrical resistivity and partial levitation over a magnet, which looked similar to properties exhibited by superconductors. "I think things are pretty decisively settled at this point," says Inna Vishik, a condensed-matter experimentalist at the University of California, Davis. The LK-99 saga began in late July, when a team led by Sukbae Lee and Ji-Hoon Kim at the Quantum Energy Research Centre, a start-up firm in Seoul, published preprints claiming that LK-99 is a superconductor at normal pressure and temperatures up to at least 127C (400 kelvin). All previously confirmed superconductors function only at extreme temperatures and pressures. The extraordinary claim quickly grabbed the attention of the science-interested public and researchers, some of whom tried to replicate LK-99. Initial attempts did not see signs of room-temperature superconductivity, but were not conclusive. Now, after dozens of replication efforts, many experts are confidently saying that the evidence shows LK-99 is not a room-temperature superconductor. The South Korean team based its claim on two of LK-99's properties: levitation above a magnet and abrupt drops in resistivity. But separate teams in Beijing, at Peking University and the Chinese Academy of Sciences (CAS), found mundane explanations for these phenomena. Another study, by US and European researchers, combined experimental and theoretical evidence to demonstrate how LK-99's structure made superconductivity infeasible. And other experimenters synthesized and studied pure samples of LK-99, erasing doubts about the material's structure and confirming that it is not a superconductor, but an insulator. The only further confirmation would come from the Korean team sharing their samples, says Michael Fuhrer, a physicist at Monash University in Melbourne, Australia. "The burden's on them to convince everybody else," he says. Perhaps the most striking evidence for LK-99's superconductivity was a video taken by the Korean team that showed a coin-shaped sample of silvery material wobbling over a magnet. The team said the sample was levitating because of the Meissner effect -- a hallmark of superconductivity in which a material expels magnetic fields. Multiple unverified videos of LK-99 levitating subsequently circulated on social media, but none of the researchers who initially tried to replicate the findings observed any levitation.

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